The flare exploded in the supporters' section and appeared to leave several fans injured.
Tensions ran high prior to AEK Athens and Ajax's UEFA Champions League match Tuesday at Athens Olympic Stadium.
As fans traded vocal barbs, a man was pictured throwing a firework into the stands of visiting Ajax supporters.
The flare set off an explosion. Several were photographed as being bloodied from the incident.
Police were pictured entering the stands working to control the scene. The game still kicked off at its scheduled time despite the chaos, and Ajax had the last laugh, securing a 2-0 win that punched its ticket to the Champions League knockout stage for the first time in 13 years.